Abstract

The purpose of this study is to investigate the cell interactions and mechanisms that limit anti-tumor CD8 T cell responses in tumor microenvironments.

We used cutting-edge multiphoton imaging of immunoevasive tumors to determine the behavior and anti-tumor responses of CD8 T cells. This was achieved by adoptively transferring T cells into a modified MMTV-PyMT breast tumor model (mouse mammary tumor virus promoter-driven, polyoma middle T antigen-induced luminal breast cancer). In this model, tumor cells expresses ovalbumin as tumor-antigen and mCherry as a fluorescent markers, thus allowing immunologic assessment and intravital imaging of T cell activity against tumor cells in and ex vivo. Furthermore, we were able to investigate the effect of antigen-presenting immune cells and immunomodulatory treatment regimens on anti-tumor T cell activity in and ex vivo.

In this work we found that newly arriving tumor-specific T cells undergo defined phases of maturation within the tumor microenvironment. First, after arriving to the tumor site, T cells engage in close interactions with antigen-presenting cells and exhibit reduced motility and rounded morphology. After this initial phase, T cells dramatically increase motility and survey the tumor microenvironment, however they do not form stable interactions with other cell types and further do not engage tumor cells. In contrary to other publications, swarming T cells do not express surface markers associated with exhaustion or anergy, and exhibit extensive in situ proliferation and ex vivo CTL activity. The capability of tumor CD8 T cells to kill targets ex vivo conflicts with the absence of tumor cell death in vivo. Additionally, individual immune cell populations isolated from tumors or tumor culture supernatants do not exhibit T cell suppressive capability when added as supplements to killing assays.

Our results define sequential fates for T cells entering tumor microenvironments, suggesting that tumor-infiltrating lymphocytes exist in distinct ‘states’ depending on the extent to which tumor antigen-presenting cells have matured them. Further, T cell suppression appears to be a local phenomenon and cannot be achieved by soluble factors or a specific cell type alone. This may suggest that combinations of therapies that individually target the different maturation states are required to achieve therapeutic effects.